Pressure sensitive adhesives are widely used for a multitute of purposes, such as various sealing and attachment purposes. Pressure sensitive adhesives may be formed as as an adhesive layer supported by a nonadhesive foundation material, which foundation may be a flexible carrier or a rigid body. Pressure sensitive adhesives may also be formed as double-faced systems, for instance as a flexible or rigid carrier faced On both sides with an adhesive layer. Such a carrier may itself be nonadhesive, although for certain applications, discussed in more detail below, the carrier supporting the two adhesive layers has itself some pressure sensitive adhesive qualities. Pressure sensitive adhesive layers vary widely as to thickness, from a fraction of a mil to much greater thicknesses.
Pressure sensitive adhesives formed substantially from acrylate copolymers are well known in the art. For instance, certain acrylate copolymers, that are both normally tacky and yet possess a reasonable degree of internal strength, are disclosed in U.S. Pat. No. Re 24,906 (Ulrich) which reissued from original U.S. Pat. No. 2,884,126 in December, 1960. Such copolymers are comprised of certain non-tertiary acrylic esters copolymerized with monomers having strongly polar groups, for instance acrylic acid, methacrylic acid, acrylamide, methacrylamide, itaconic acid, and the like. Such non-tertiary acrylic esters are generally acrylic acid esters of non-tertiary alkyl alcohols, such alcohols having up to 14 carbons, and further having an average of from 4 to 12 carbons. Small amounts of other monomers may also be incorporated into such polymers. Such adhesive polymers are typically applied to a flexible sheet material for use.
Photopolymerization of pressure sensitive adhesive materials of the acrylate copolymer type is generally known. While such acrylate copolymers may be prepared by solution or emulsion polymerization techniques, there are manufacturing disadvantages of preparing the polymers in such fashion. If solution polymerization is used, the solvent must be removed, typically by drying it off at elevated temperatures, which involves the employment of significant processing equipment and may create environment and safety hazards. Emulsion polymerization also requires the polymer to be separated from the liquid used, that is, the water. While the emulsified polymer may be separated by precipitation, rather than drying off the water, the resultant polymer will contain the surfactant used in the polymerization, which may be detrimental to the end-use, and additional processing steps beyond the polymerization itself are required. Photopolymerization of the monomers neat, without any diluent that needs to be removed after the polymerization, provides processing advantages in the manufacture of pressure sensitive adhesives. For instance, Belgium Patent No. 675,420, published in May of 1966, discloses a process for the fabrication of adhesive tapes comprising flexible carriers and polyacrylic adhesive coatings thereon by polymerizing the monomers directly on the carrier using photopolymerization through exposure of the monomers, containing a photoinitiator, to ultraviolet radiation. The liquid acrylic monomer(s), which may be thickened, are applied to the carrier using a doctor blade or roller coating, or by spraying, and then polymerized, avoiding any post-polymerization removal of diluent or other extraneous material. Exemplified is a typical pressure sensitive formulation, a mixture of ethyl hexyl acrylate and acrylic acid, that is coated onto a cellulose acetate sheet and is passed into a tunnel, from which oxygen has been excluded, the upper wall of which is formed of quartz glass, and is irradiated with a source of ultraviolet radiation through such upper wall. The source of ultraviolet radiation used is a high pressure mercury vapor lamp. A mercury bath is used to avoid an overly high temperature that would otherwise ensue from the exothermic polymerization reaction. Also disclosed in this patent is: the use of a temporary carrier that is an endless belt, which does not become incorporated into the end product; the use of the photopolymerization technique as only a portion of the polymerization process, for instance the polymerization being completed in an oven; and the preparation of tapes having a pressure sensitive adhesive layer on both sides.
It is also known to prepare a pressure sensitive adhesive tape by first forming a prepolymer of at least some of the monomers to be used, and then coating such prepolymer to the desired thickness before completion of the polymerization. For instance, U.S. Pat. No. 3,729,338 (Lehmann et al.) discloses the preparation of a low molecular weight, spreadable composition which is applied to the support material, to which composition may be added a small amount of catalyst and/or a polyfunctional cross-linking monomer, prior to the completion of the polymerization by heat curing. Such low molecular weight polymer, or prepolymer, is thicker or more viscous than the liquid monomers and hence is more easily applied as a coating to the support material before polymerization is completed.
Pressure sensitive adhesive tapes that are both relatively thick and are comprised of layers of acrylate esters have been prepared using photopolymerization techniques. For instance, U.S. Pat. No. 4,223,067 (Levens) describes the preparation of a pressure sensitive adhesive tape that may be from 0.2 to 1.0 mm thick. Such tape is comprised of a pressure sensitive adhesive matrix which further contains glass microbubbles, which optionally may have one or both of its sides coated with an unfilled pressure sensitive adhesive. The photopolymerization is accomplished by exposure to ultraviolet BL and BLB lamps. Where a laminate of both filled and unfilled layers was prepared, the separate polymerized layers were laminated together using a pressure roll.
The use of BL and BLB lamps in particular for the photopolymerization of acrylate ester pressure sensitive adhesives is discussed in detail in U.S. Pat. No. 4,181,752 (Martens et al.). Such lamps emit ultraviolet radiation that peaks at about 351 nm, and the energy emitted at ultraviolet radiation wavelengths shorter than 300 nm is less than 10 percent of that in the 300 to 400 nm range, and at a distance of about 3 inches have a light intensity in the 300-400 nm wavelength band of about 0.337 milliwatts/cm.sup.2. An unduly slow polymerization is noted as being disadvantageous to the manufacture of the pressure sensitive adhesive.
The present invention is directed to a process for preparing a pressure sensitive adhesive at least in part by photopolymerization of acrylate esters or prepolymers thereof, wherein by virtue of the source of ultraviolet radiation employed, the polymerization is completed at a faster rate without loss of performance properties of the end product. The present invention is also directed to such a process for the preparation of a pressure sensitive adhesive, particularly a relatively thick pressure sensitive adhesive, wherein many of the performance properties of the end product are improved.